Field of the Invention
The present invention relates to a novel initiator for photopolymerization of cationically polymerizable organic material. More particularly, the present invention relates to an initiator for photopolymerization which can be added to cationically polymerizable organic material in a suitable amount and can polymerize the cationically polymerizable material with irradiation of active photoenergy, thereby converting the organic material into high molecular weight state.
Disclosure of the Prior Art
Various onium salts such as diazonium salts, iodonium salts, sulfonium salts, etc., and salts of metallocene complexes are known as initiator for photopolymerization of cationically polymerizable material [for example, Annual Reviews Materials Science, 13, 173-190 (1983); Journal of Radiation Curing, 13 (10), 26-32 (1986); Japanese Patent Laid-open No. 151,996/75; Japanese Patent Laid-open No. 15,261/81; and Japanese Patent Laid-open No. 210,904/83].
The working mechanism of these known initiators in photopolymerization are as follows:
In case of using various diazonium salts, iodonium salts and sulfonium salts, a Bronsted acid is generated at first by irradiation of U.V. light in accordance with the Scheme I, II and III. ##STR1##
The generated Bronsted acid can react with the cationically polymerizable organic material and the polymerization proceeds in accordance with Scheme IV, whereby molecular chain of polymer grows. ##STR2##
On the other hand, in case of using a salt of metallocene complex, a Lewis acid is generated with irradiation, and an insertion of monomer occurs at the Lewis acid, whereby molecular chain of polymer grows. ##STR3##
A process of initiation of polymerization with conventional photoinitiators comprises a generation of Bronsted acid or Lewis acid by irradiation of U.V. light. The diffusion of these acids in monomer system and the insertion of monomer into coordinate bond of these acids are rate-determining step of polymerization. The diffusion of these acids or the insertion of monomer is slower than diffusion of radical in photoradical polymerization and accordingly the polymerization with the conventional photoinitiators is slower than photoradical polymerization.
The maximum absorption wavelength of the conventional photoinitiators is within ultraviolet shorter wavelength zone of not higher than 300 nm. when a ray having the wavelength exceeding the zone mentioned above is used (for example, i ray 365 nm; g ray, 436 nm; photolithography with a positive film), the acids which are essential for initiation of polymerization are not generated, or even if the acids are generated, their quantum efficiency is very low.
Therefore, when they are used for polymerization of colored cationical photocuring compositions such as epoxy resin paint, etc.; photocuring compositions usable for sealing, closing adhesion; industrial materials such as photoresist material which is used for preparation of wiring pattern on semiconductor or printed circuit board, they must be used in much amount, or they must be used together with a sensitizer such as aromatic condensed polynucleus compounds (for example, anthracene), thioxanthone, benzoin alkyl ether, alkoxy acetophenone, etc. in order to vest the resulting mixture with photosensitivity in longer wavelength zone or improve reaction rate.
Even if the recipe mentioned above is adopted, insufficient thick film curability is often observed and the cured material which does not have desired physical properties is often obtained. In view of these factors, a composition comprising an initiator which has both of photocuring and thermocuring has been developed. Such photoinitiators are disclosed in for example, Japanese Patent Laid-open No. 210,904/83 and No. 223,020/86.
However, as the addition amount of photoinitiator increases, the amount of photodecomposition product increases, and as a result, unpolymerized low molecular-weight compounds may be included in the obtained cured material and remain therein. The use of sensitizer leads also the similar results. When the cured materials including the unpolymerized low molecular weight compounds are used in industrial field, there exist some defects such as rapid deterioration of cured material, corrosion of mother material to be coaded, etc.
An adoption of post-heat curing step is not preferable, because the period of heat curing step is dependent to temperature of mother material to be coated and temperature of atmosphere at which the heat curing is carried out and accordingly the control of period of heat curing is difficult, especially in automatic operation system. In addition, the adoption of post-heat curing step leads an increasment in number of step and time and accordingly injures the merits obtainable from use of photoinitiator and/or sensitizer such as rapid curing, improvement of productivity, etc.